Effect of irrigation water source and post-harvest washing treatment on the microflora of alfalfa and mung bean sprouts

Abstract

Alfalfa and mung bean sprouts were grown using municipal water or water generated from an aquaponics system. After seven days of growth, sprouts were harvested, separated into 25-g portions and assigned to one of four different treatment groups. With the exception of the control treatment, all treatments involved washing for 1 min in 500 mL of solution, followed by draining and sampling sprouts for selected microorganisms. Post-harvest washing treatments consisted of the following: 1) no wash control; 2) municipal water wash, pH 6.58; 3) chlorine wash at 200 ppm, pH 9.15; and 4) organic acid wash prepared at a 1% concentration and pH 1.52 from a stock solution that contained ≤35% sulfuric acid, ≤10% citric acid, and ≤5% phosphoric acid by weight. Overall, the numbers of total aerobic microorganisms (TAM), Enterobacteriaceae (EB), total coliforms (TC) and yeast and mold (YM) recovered from alfalfa or mung bean sprouts grown in aquaponics water were higher than the number of TAM, EB, TC and YM from sprouts grown in municipal water. Alfalfa and mung bean sprouts grown in municipal water and washed post-harvest with either municipal water or chlorine had fewer TAM (1.1–1.4 log CFU/g lower) and YM (0.6–1.3 log CFU/g lower) than unwashed alfalfa or mung bean sprouts. When sprouts were grown in aquaponics water and washed post-harvest with municipal water, they had similar numbers of TAM, EB, TC and YM to control sprouts. Washing sprouts with chlorine had little or no effect on levels of TC, and in some cases, sprouts treated with a chlorine wash had higher numbers of TC than control sprouts. This likely occurred because the primary biocidal activity of the 200 ppm chlorine solution was directly related to pH (>pH 9.0) and not hypochlorous acid, which may have created a favorable environment for coliform regrowth. The post-harvest washing treatment that resulted in the greatest reduction in counts of microorganisms on sprouts was the organic acid wash. When compared to the control treatment for alfalfa sprouts, the organic acid wash reduced populations of TAM, EB, TC and YM recovered from alfalfa sprouts by greater than 99.97%. Similarly, the organic acid wash reduced populations of TAM, EB, and YM by greater than 99% on mung bean sprouts when compared to populations recovered from the control treatment. The results of the present study demonstrated that aquaponics water provided adequate nutrients for growth of sprouts and microorganisms. Furthermore, this study showed that sprouts contain a significant number of microorganisms that are difficult to reduce with post-harvest wash treatments, supporting the notion that multiple hurdle technology may be the only solution for lowering final counts on sprouts to levels below 3 log CFU/g.